Method and apparatus for adjusting purity



n- 1969 E. c. MACINTYRE, JR, ETAL 3,421,043

METHOD AND APPARATUS FOR ADJUSTING PURITY Filed April 28, 1967 Sheet of2 DYNAMIC DEFLECTION CONVERGENCE WAVE APPARATUS GENERATOR- FIG! J /1VIDEO R SIGNAL 5 16 SOURCE I G 20 28 f "I IO 26 I 38 34 so n4 5 FIG? \P\M ([HTL: H0 3 A I12 1 n2 B" INVENTORS ERNEST C. MACINTYRE ROBERT B-HANSEN BY t 20401 ATTORNEYS.

United States Patent 3,421,043 METHOD AND APPARATUS FOR ADJUSTING PURITYErnest C. Macintyre, Jr., Villa Park, and Robert B. Hansen, ArlingtonHeights, Ill., assignors to Motorola, Inc., Franklin Park, 111., acorporation of Illinois Filed Apr. 28, 1967, Ser. No. 634,529

US. Cl. 315-13 Int. Cl. H01j 29/50 11 Claims ABSTRACT OF THE DISCLOSUREBackground of the invention The cathode ray tube generally used in colortelevision receivers is of the type which has a screen composed of amultiplicity of fluorescent dots arranged in geometrically similargroups, and a shadow mask mounted between the screen and a plurality ofelectron guns. Each of the different fluorescent dots in each grouppossesses a response characteristic and is capable of emitting light ofa particular primary color when impinged by an electron beam. The gunsare arranged with respect to one another so that they individuallydirect a beam through the common apertures of the shadow mask fromdifferent angles so the beams impinge associated ones of the fluorescentdots. The beams are swept across the screen by horizontal and verticaldeflection apparatus to produce a multi-colored reproduction of atelevised scene.

Because of imperfect placement of the various elements in the cathoderay tube, such as the guns and the mask, the cathode ray beams mayundesirably impinge other than their associated fluorescent dots. Inorder to preclude this, it is common practice to provide a set of puritymagnets or the like to simultaneously shift the axial paths of thecathode ray beams. This may be done by turning on and deflecting onlyone beam, such as the one associated with the blue portion of the image,and adjusting the magnets for the best blue field, or it may be done byturning on all the beams and adjusting for the best White field. It isdesirable to permanently adjust the magnets at the factory where theearths magnetic field is in a given direction and of a given magnitude.If, then the color television receiver is used in another location wherethe earths magnetic field is different, the axial paths of the cathoderay beams may be affected to such an extent that they impinge on otherthan their associated fluorescent dots which adversely affects thepurity of the reproduced picture.

Since only portions of the areas of the dots are illuminated, the idealprocedure would be to adjust the purity magnets so that the cathode raybeams illuminate precisely the center portions of their associated dotsover the entire screen. If the earths magnetic field changes when thereceiver is used in a new location, the beam impingements move outwardlytoward the peripheries of their fluorescent dots but are still containedtherein so that there is no loss in purity. However, with presentpractices, such central placement of the beams is purely a matter ofchance because if, for example, -purity 3,421,043 Patented Jan. 7, 1969is adjusted when the blue gun is on, the technician will simply adjustthe magnets for the best blue field. With this procedure, the blue beammay impinge the peripheries of the blue dots throughout the screen and,therefore, it is quite possible that such beam will impinge other thanits associated dots by a slight change in the earths magnetic field.Another possible effect of such an alignment procedure is that certainareas of the screen will have the central portions of the blue dotsilluminated while in other areas, the peripheral portions of such dotswill be illuminated. In such case it is desirable to effect a compromiseso that there is at least some tolerance or guard band over the entirescreen.

Summary of the invention It is, therefore, an object of this inventionto adjust purity in a given receiver location and thereby optimizepurity for all locations.

Another object is to provide a maximum tolerance or guard hand betweenthe areas of fluorescent dots illuminated by an associated electron beamand fluorescent dots not associated therewith.

In a specific form of the invention a cathode ray tube to be aligned isof the type which has a screen covered by fluorescent dots of aplurality of different color response characteristics. The tube containsa shadow mask having a multiplicity of systematically arranged aperturesthrough which a plurality of cathode ray beams pass along differentangularly related paths to impinge upon and illuminate a portion of theareas of associated fluorescent dots. The circumferential spacing orguard band between such areas and non-associated fluorescent dots overthe entire screen is to be maximized by rotating, or deflecting in agiven path, at least one of the beams about its axial path to subjectother than its associated dots to be illuminated thereby. The axial pathof the one beam is then shifted by static purity adjustment in adirection to minimize the total screen illumination of the fluorescentdots not associated with the one beam. When the rotation is thenterminated the guard band of beam landing on the intended phosphor dotsis at a maximum over the entire screen.

Description of the drawings FIG. 1 is a plan view representation of atri-gun cathode ray tube with the beam shift apparatus, convergenceassembly and deflection yokes shown in block form;

FIG. 2 schematically illustrates the convergence assembly of FIG. 1;

FIG. 3 illustrates the purity magnets which may comprise the beam shiftapparatus in FIG. 1;

FIG. 4 schematically illustrates the screen of the cathode ray tube ofFIG. 1 with enlarged fragmentary portions and illustrative apparatus forrotating the cathode ray beams;

FIG. 5 is a block diagram of the type of circuitry which may be used toenergize the beam rotating apparatus of FIG. 4;

FIG. 6 is an enlargement of a color dot triad illustrating the effect ofrotating one of the cathode ray beams; and

BIG. 7 illustrates the effect of shifting the axial path of the cathoderay beam while it is rotating.

Detailed description Referring now to the color television receiver ofFIG. 1, the tri-gun cathode ray tube 10 has a screen 12 provided with amultiplicity of fluorescent dots arranged in groups of three, andcapable respectively of producing light of a different primary colorsuch as red, blue and green when impinged by an electron beam. In backof and spaced from the screen 12 there is a shadow mask 14 having anaperture for and in substantial alignment with each group or triad offluorescent dots on the screen 12. The tube has a plurality of electronguns such as a blue gun 16, a red gun 18, and a green gun 20, equal innumber to the number of primary colors in which the image is to beproduced. When the blue beam 22, the red beam 24 and the green beam 26from these guns properly converge at the shadow mask 14, theysimultaneously pass through each aperture from different directions andimpinge upon associated fluorescent dots so as to produce blue, red andgreen light. The electron beams are modulated in intensity under thecontrol of color representative video signals derived from a source 28.Also associated with the cathode ray tube are a set of horizontal andvertical deflection yokes indicated generally by block 30 which areenergized by a deflection wave generator 32 to scan the screen 12. Aconvergence assembly 34 shown in some detail in FIG. 2 is energized bydynamic convergence apparatus 36 to converge the three electron beams atall points of the shad-ow mask 14. Beam shift apparatus 38 may comprisea pair of magnets as shown in FIG. 3 and as explained in more detailhereinafter, serve the purpose of simultaneously shifting the axialpaths of the three cathode ray beams.

Referring now to FIG. 2, the convergence assembly 34 includesconvergence magnets 40, 42 and 44 each consisting of a pair of externalpole pieces 46 and 48 and a pair of internal pole pieces 50 and 52 toincrease the effectiveness of the magnets. (The letters a and b on thereference numerals indicate similar parts on magnets 42 and 44.) Thecoils 54 and 56 are energized by the dynamic convergence apparatus 36(FIG. 1) to converge the electron beams 16, 18 and 20 to all points onthe screen 12. Also associated with each convergence magnet is a staticconvergence magnet 58 to independently displace the electron beams 22,24 and 26 radially to insure that they are initially converged at aselected position on the shadow mask 14. However, due to im perfectlocation of the elements within the cathode ray tube, such as theelectron guns 1620 or the mask 14, the central axis of the guns may notbe aligned with the center of a given dot group behind the aperture atsuch selected position. This may cause the beams to impinge upon otherthan associated dots.

In order to compensate for this, the beam shift apparatus 38 of FIG. 1is provided which may consist of a pair of purity magnets 60 and 62 asshown in FIG. 3. They are magnetized in a manner such that when they arerotated with respect to each other to a position where the flux fieldsare aiding, the beams are displaced the greatest amount, and when theyare turned so that the flux fields are subtracting, the beams are notaffected. Any desired degree between zero and maximum may be obtained byrotating one magnet with respect to the other. The direction of movementmay be altered as desired by rotating both magnets together. Thus, byproperly adjusting the position of the magnets, the axial paths of thethree beams may be simultaneously shifted so that they land only ontheir associated dots and thereby provide optimum purity. The use ofmagnets is merely illustrative and other known means to simultaneouslyshift the paths of three beams may be employed. Preferably thedeflection yokes are axially movable to aid in purity adjustments. Thispurity adjustment procedure may be effected in a variety of ways, butgenerally, only one of the guns, such as the blue gun 16, is turned on.As the blue beam 22 is deflected across all of the associated dots onthe screen 12, by the deflection wave generator 32, the magnets areadjusted until there is no red and green showing. Since the three beamsare similarly affected by the magnets, such adjustment should optimizepurity for all three.

Referring now to FIG. 4, the screen 12 comprises a multiplicity oftriads of red, blue and green color fluorescent dots, the color responsecharacteristics being indicated as R, B and G with a pair of enlargedtriads shown in opposite corners. In the following description, althoughonly one triad at a time may be considered, it should be remembered thatthe beams will illuminate other triads at the same instant in time dueto the beam having a size larger than the apertures in the shadow mask14.

Suppose after adjusting the magnets the blue beam 22 illuminates an area63 in the upper left hand corner of the screen 12, and an area 64 in thelower right hand corner thereof. As far as the technician who adjuststhe magnets can see, the purity is optimized because although the bluebeam 22 lands near the periphery of its associated dot in the upper lefthand corner, it still appears blue. If, with this setting, thetelevision receiver is used in a different location where the intensityand/or direction of the earths magnetic field is different, the bluebeam 22 may move to the right to undesirably impinge part of the reddot. Since the purity magnets and the earths magnetic field affect allthree beams simultaneously, the red and green beams 18 and 20 will movein the same direction by corresponding amounts.

It is desirable, therefore, that when the technician is adjusting forpurity, he should adjust it in such a way that the blue beam 22 landsprecisely in the center of the blue dot throughout the screen 12. Thenwhen the set is moved to a different location, the blue beam, and thusthe other beams, will still land on their associated dots. Even in thecase shown in FIG. 4, where the blue beam (and the green and red beams)cannot be placed in the central portion of its dots throughout thescreen, if the axial paths of the beams were moved slightly to the leftby the beam shift apparatus 38, they would no longer be centered in thelower right hand corner of the screen 12, but, more important, theywould not land on the periphery of the dots in the upper left handcorner. Now, a change in the earths magnetic field will shift the beamsbut since there is some guard band or circumferential spacing betweenthe area illuminated by an associated beam and a non-associated dot,there is less possibility of a loss in purity. The object then is tocenter the electron beam on their associated dots throughout the screen,to thereby provide the maximum possible guard band for each dot, or ifthat is not possible, at least effect a compromise so that thecircumferential spacing between the areas illuminated by an associatedbeam and fluorescent dots not associated with that beam is maximizedover the entire screen. This is accomplished by using the method of theinvention which includes rotating the beam to make it more susceptibleto impinge on other than associated dots and then adjusting the beamshift apparatus 38 to minimize this latter impingement.

Exemplary apparatus for providing such rotation is shown in FIG. 5 andincludes an oscillator 65, three phase shifters 6670 to produce threesignals having a selected phase relationship such as 0, 60 and 120,which are then amplified by amplifiers 7276 and applied to three pairsof windings 78-88 having the junction between each pair grounded. Thusthe windings will have the following phase relationship: winding 78 at0, winding 80 at 180, winding 82 at 60, winding 84 at 240, winding 86 atand winding 88 at 300.

As shown in FIG. 4, the windings are respectively wound on iron cores90400 in a direction to cause the flux to be as indicated by the arrows.The cores are mounted within an annular frame 102 encircling the portionof the cathode ray tube 10 adjacent the shadow mask 14 (FIG. 1). Theframe has a non-conductive innerpart 104 and a conductive strap 106 tohelp contain the magnetic field created by these windings and to providea convenient common ground connection. By using three phase analysis, itcan be shown that a beam on a path to impinge the screen 12 will berotated at a frequency determined by the frequency of the signaldeveloped by oscillator 63 with the radius of rotation determined by theamplitude of the signals in the windings. The six pole scheme is merelyillustrative and either more or less poles or other means to develop arotating field such as electrostatic means may be utilized and yet comewithin the scope of the invention. Alternately instead of encircling theportion of the tube adjacent the shadow mask, as shown, the frame 102may be placed at any convenient location all the way back to theelectron guns 16-20.

To understand the effect of the rotating field, reference is made toFIG. 6 which illustrates the fluorescent dot triad in the upper lefthand corner of the screen 12 of FIG. 4 on an enlarged scale. It isassumed here that the blue beam 22 initially impinges and illuminatesthe upper right hand area of the blue dot. When the rotating field isapplied, the beam moves in a circular path indicated by numeral 110 withthe radius thereof determined by the amplitude of the signals in thewindings 78-88. At one instant of time during such movement, the bluebeam will land in a position to illuminate an area 112 (verticallyhatched (entirely within the blue dot and at another instant, itilluminates an area 114 (horizontally hatched) where a portion of thered dot is also illuminated. The technician seeing that the blue beam 22is impinging other than its associated dot, will adjust the magnets 60and 62 shown in FIG. 3 so as to move the rotating blue beam down and tothe left to the position shown in FIG. 7 where the respectiveilluminated areas are 112 and 114', both being within the blue dot. Insuch position, the blue beam 22 will rotate along the circular path 110and illuminate only areas within the blue dot. When the rotation isterminated, the blue beam 22 will land precisely in the center of theblue dot. However, it will be noted that such adjustment of the magnets60 and 62 will shift the blue beam so that it will now impinge the lowerleft hand side of its associated blue dot in the lower right hand cornerof the screen 12 of FIG. 4. Thus, instead of moving the blue beams asfar over as indicated in FIG. 7, the final position should be somewherebetween that shown in FIGS. 6 and 7 to elfect a compromise.

Since the magnets simultaneously affect the axial paths of all threebeams, what has been said with respect to the areas illuminated by theblue beam 22 is similarly applicable to the red and green beams 24 and26. It is, of course, possible that when the static convergence magnets58, 58a and 58b of FIG. 2 are adjusted to converge the beams at a givenposition on themask 14, and the dynamic convergence apparatus 36converges the beam over the entire mask, the placement of the guns 16-18and the mask is such that each beam illuminates the same area of theirassociated dots over the entire screen. In such case after rotating andshifting the beams, they will illuminate the center of their associateddots. In any case whether or not the elements are ideally placed, thebeams are shifted, while rotated in a direction and by an amount suchthat the tolerance or guard band between the area of an associated dotilluminated by an associated beam and a non-associated dot is maximizedover the entire screen 12.

Although an impure blue field is more easily detected, purity could beadjusted with either the red or green guns 18-20 on and maximizing thered or green fields respectively. Or, all three guns 1620 may be turnedon simultaneously rotating the beams 2226, and shifting the axial pathsof the beams by the permanent magnets 60 and 62 of FIG. 3 to produce awhite field on the screen 12.

Preferably in order that this rotating field be more perceptible thatis, easier for the technician to see the beam movement from blue togreen to red, etc., the frequency of rotation of the beam should be afew cycles per second. Since the frame frequency is 60 cycles persecond, that is, a given dot will be energized every 60th of a second,it is desirable that the oscillator be a few cycles off such frequencyor a multiple thereof so that a stroboscope effect does not occur. If,for example, the frequency of the signal from oscillator 65 is 62 cyclesper second, the beam will appear to rotate at a 2 cycle rate. When themagnets are improperly adjusted, the technician will see color flashingover those areas of the screen where the beam is impinging other thanassociated dots. For example, if the rotation was clockwise, he wouldsee a flash of blue, followed by a flash of blue-green, then possiblyall three colors, etc. As the deviation from the frame frequencyincreases, the speed of rotation is also increased. The rotation of abeam may also be viewed as an effective way to increase the diameter ofits landing. It should be remembered, however, that at each instant oftime, a number of triads will be impinged due to the diameter of eachbeam being large enough to pass through several apertures in the shadowmask 14. Thus, landing means the landing of one beam as it passesthrough a given aperture. If the area of landing of a non-rotating beamis a given percent of the area of a fluorescent dot, when the beam isthen rotated the effective area of the beam landing will increaseproportional to the amplitude of the signals applied to the windings7888 of FIG. 4, in turn proportional to the amplitude of the signal fromoscillator 65. Assuming perfect cathode ray tubes could be manufactured,it would be desirable to select the amplitude of the signals in thewindings such that the circular path of the rotating beam is largeenough that the beam landing has an area substantially equal to the areaof a fluorescent dot. In such case when the technician adjusts thepurity magnets 60 and 62, the blue beam will entirely fill up each bluedot to thereby provide the best blue field and at the same time anoptimum guard band over the entire screen 12. If any imperfections existin the cathode ray tube, however, flashing colors will appear on certainareas of the screen 12, irrespective of the adjustment of the puritymagnets 60 and 62. It is possible to utilize such a large amplitude oreven a larger one, in which case the purity magnets would be adjusted tominimize the amount of flashing. However, it is preferable to reduce theamplitude of the signals through the windings 78-88 to a point wheremost tubes will provide no flashing when the purity magnets are properlyset. In such case the technician has the simple task of turning on therotating field and adjusting the magnets 60 and 62 until he sees a bluefield.

The method disclosed herein is not only useful in aligning a colortelevision receiver but may be also useful in incoming inspection todetermine the quality of color cathode ray tubes. In such case theamplitude of the signal from oscillator 65 would be selected to providea given radius of the rotating beam. If the technician could not adjustthe purity magnets 60 and 62 to cause the beam to impinge only itsassociated dots, such tube would be rejected. It may be seen that as thesignal amplitude is decreased, the radius of the beam rotationdecreases, and less tubes will be rejected, but the average size ofguard bands would probably be less.

The invention is not limited to circular rotation of the beams, butrather may involve a complex rotation, for example, to compensate forbeam shift due to temperature changes in the cathode ray tube elements.

We claim:

1. In the method of aligning a cathode ray tube of the type which has ascreen covered by fluorescent dots of a plurality of different colorresponse characteristics, and which tube contains a shadow mask having amultiplicity of systematically arranged apertures through which aplurality of cathode ray beams pass along different angularly relatedpaths to impinge upon and illuminate a portion of the areas ofassociated fluorescent dots, the method of maximizing thecircumferential spacing between such areas and non-associatedfluorescent dots over the entire screen, which method includes the stepsof: rotating at least one of said beams about its axial path towards thescreen to subject other than its associated dots to be illuminatedthereby, shifting said axial path in a direction to reduce theillumination of other than the fluorescent dots associated with said onebeam, and terminating the rotation of said one beam.

2. The method of aligning a cathode ray tube according to claim 1wherein the axial path of said one beam is continually deflected so thatduring one complete scan of the screen it impinges substantially all ofits associated dots, the rotation of said one beam being performed Whilethe one beam is being deflected.

3. The method of aligning a cathode ray tube according to claim 1wherein said plurality of different color response characteristics ofthe fluorescent dots and the plurality of cathode ray beams numberthree, simultaneously rotating said three beams about its axial path,shifting said axial paths in a direction to produce a maximum whitefield on the cathode ray tube screen.

4. The method of aligning a cathode ray tube according to claim 1wherein the rotation of said one beam is circular.

5. The method of aligning a cathode ray tube according to claim 1wherein said one beam is vertically deflected across the cathode raytube screen at a 60 cycles per second rate, rotating said one beam in acircular motion simultaneous with the deflecting of the axial path at afrequency at least one cycle per second removed from said 60 cycles persecond rate.

6. The method of aligning a cathode ray tube according to claim 1wherein the rotation increases the elfective area of impingement by theone beam passing through a given aperture in the shadow mask, the radiusof rotation of the beam selected to cause the effective area to be nomore than the area of a fluorescent dot.

7. In the method of aligning a cathode ray tube of the type which has ascreen covered by fluorescent dots of a plurality of different colorresponse characteristics, and which tube contains a shadow mask having amultiplicity of systematically arranged apertures through which acorresponding plurality of cathode ray beams is passed along differentangularly related paths to impinge upon associated fluorescent dots,with a given beam subject to illuminate approximately the central areaof some of its associated dots and a peripheral area of other of itsassociated dots, the method of maximizing the number of dots havingareas intermediate said peripheral and central areas illuminated by thegiven beam, which method includes the steps of: continually deflectingthe axial path of said given beam so that during one complete scan ofthe screen it impinges substantially all of its associated dots,rotating said given beam in a circular motion about such path toincrease its effective size at the screen to subject other than itsassociated dots to be illuminated by said given beam to thereby indicatethe number of dots having peripheral areas illuminated by said givenbeam when it is not rotating, shifting said axial path in a direction tominimize the illumination of other than the dots associated with saidgiven beam over the entire cathode ray tube screen and thereby reducethe number of associated dots having a peripheral area illuminated bysaid given beam and increase the number of dots having approximately thecentral area illuminated thereby, and terminating the rotation of saidgiven beam.

8. The method of aligning a cathode ray tube according to claim 7wherein said plurality of different color response characteristics ofthe fluorescent dots and the plurality of cathode ray beams numberthree, statically converging said three beams at a selected position onthe shadow mask prior to rotation of said given beam.

9. The method of aligning cathode ray tube according to claim 7 whereinthe axial path of said given beam is vertically deflected at a cycle persecond rate, and wherein the rotation of said given beam is at afrequency at least one cycle per second removed from an integralmultiple of 60 cycles per second.

10. In the method of determining the quality of a cathode ray tube ofthe type which has a screen covered by fluorescent dots of a pluralityof different color response characteristics, and which tube contains ashadow mask having a multiplicity of systematically arranged aperturesthrough which a plurality of cathode ray beams pass along differentangularly related paths to impinge upon and illuminate a portion of theareas of associated fluorescent dots, the method of determining whetherthe circumferential spacing between such areas and nonassociatedfluorescent dots meets a predetermined standard, which method includesthe steps of: rotating at least one of said 'beams about its axial pathtowards the screen to subject other than its associated dots to beilluminated thereby, shifting said axial path in a direction to reducethe illumination of other than the fluorescent dots associated with saidone beam, and rejecting as being of insufficient quality those cathoderay tubes which have a predetermined degree of illumination ofnon-associated dots during such rotation.

11. In combination with a cathode ray tube of the type which has ascreen covered by fluorescent dots of a plurality of different colorresponse characteristics, and which tube contains a shadow mask having amultiplicity of systematically arranged apertures through which aplurality of cathode ray beams pass along different angularly lightedpaths to impinge upon and illuminate associated fluorescent dots, meansfor manually adjusting color purity by shifting the axial path of atleast one of the beams, purity adjustment apparatus comprising, fieldproducing means adapted to be mounted adjacent the path of the one beamfor shifting its axial path, signal producing means coupled to saidfield producing means to provide a signal therefor of a frequency on theorder of 60 cycles per second and of an amplitude to shift the axialpath of said one beam so that the same falls on an area of the screenlarger than the area thereof impinged with said purity adjustmentapparatus disabled, whereby optimum adjustment of the means for manuallyadjusting color purity is more apparent.

References Cited UNITED STATES PATENTS 2,910,618 10/1968 Vasilevskis31513 RODNEY D. BENNETT, Primary Examiner.

C. L. WHITHAM, Assistant Examiner.

US. Cl. X.R. l.

